In recent years, a technique called CDMA has been becoming a world standard as access means in the mobile communication field. CDMA communication systems, however, have entailed a big problem called the near-far problem in which as the distance between a mobile terminal and a base station decreases, leakage power to adjacent channels increases, increasing the bit error rate and degrading the communication quality.
To overcome this problem, signal output power must be controlled in accordance with the distance between the mobile terminal and the base station. More specifically, in view of the size of the cell range covered by the base station, gain control over a wide range of 70 dB or greater in terms of gain control width must be performed at the mobile terminal transmitter. Moreover, as a feature of CDMA, extremely precise gain control is performed at any given distance from the base station. Therefore, gain control having excellent linearity with a flatness of .+-.1 dB is essential.
Furthermore, when gain is attenuated at the mobile terminal transmitter, noise figure increases; therefore, if the gain is attenuated over a wide range of 70 dB or greater at intermediate frequencies lower than about 500 MHz where the carrier signal level is low, it would become difficult to distinguish between the level of the carrier signal and the level of noise signals, resulting in a degradation of communication quality. To avoid this problem, the gain control must be performed at high frequencies higher than about 500 MHz where the carrier signal level is high enough that the level of the carrier signal is easily distinguishable from the level of noise signals.
To accomplish the gain control having excellent linearity with a flatness of .+-.1 dB over a wide gain control range of 70 dB or greater at a mobile terminal transmitter, it has been practiced in the prior art to manufacture two separate semiconductor devices for performing two different kinds of gain control, i.e., the first packaged semiconductor device for performing control in step-like manner using a point in a high frequency range at which the gain changes largely and the second packaged semiconductor device for performing continuous control using an intermediate frequency range where the gain changes linearly, and to connect the two semiconductor devices together using an external circuit. In a mobile terminal, such gain control is performed using separate semiconductor devices with built-in microcomputer logics.
The reason that the two separate kinds of gain control are performed is that gain control having excellent linearity over a wide range, for example, a flatness of .+-.1 dB over a gain control range of 70 dB or greater, has been difficult to accomplish using a single semiconductor device forming the high frequency section.
A description will be given below of amplifiers as representative semiconductor devices for performing gain control in a prior art mobile communication terminal transmitter.
FIG. 19 is a circuit diagram showing the configuration of an amplifier (semiconductor device) which performs gain control in steplike manner in a high frequency section of the prior art mobile communication terminal transmitter. With this amplifier, the gain is controlled in steplike manner using apoint where the gain changes largely.
As shown in FIG. 19, the amplifier includes a signal line 77 containing a series variable resistor 71 and connecting between an input terminal 14 as a signal input part and an output terminal 15 as a signal output part, and shunt (parallel) variable resistors 72 and 73 are connected between a ground line 76 and the input terminal 14 and output terminal 15, respectively. The ground line 76 is connected to ground GND which is a base potential part. A gain control line 75 is connected to the variable resistors 71, 72, and 73. In this amplifier, a gain control voltage application terminal 4 as a gain control voltage application part is connected to the variable resistors 71, 72, and 73 via the gain control line 75.
The variable resistors 71, 72, and 73 are constructed from field effect transistors 6, 1, and 9 whose gates are connected to resistors 7, 5, and 13, respectively. The drain of the field effect transistor 6 forming the variable resistor 71 is connected to the input terminal 14, and the source is connected to the output terminal 15. On the other hand, the drain of the field effect transistor 1 forming the variable resistor 72 is connected to the input terminal 14 via a capacitor 2, and the source is connected to the ground GND via a capacitor 3 and ground line 76. Likewise, the drain of the field effect transistor 9 forming the variable resistor 73 is connected to the output terminal 15 via a capacitor 10, and the source is connected to the ground GND via a capacitor 11 and ground line 76.
Further, the gate of the field effect transistor 6 forming the variable resistor 71 is connected to the gain control voltage application terminal 4 via the resistor 7 and gain control line 75, the source of the field effect transistor 1 forming the variable resistor 72 is connected to the gain control voltage application terminal 4 via the gain control line 75, and the source of the field effect transistor 9 forming the variable resistor 73 is connected to the gain control voltage application terminal 4 via the gain control line 75. Supply voltage VDD (about 3 V, a battery voltage itself) from a lithium battery or the like is applied to the source of the field effect transistor 6 forming the variable resistor 71, while GND potential is applied via the resistors 5 and 13 to the gates of the field effect transistors 1 and 9 forming the respective variable resistors 72 and 73.
Here, the capacitors 2, 3, 10, and 11 each act to prevent the application of a dc voltage, while the resistors 7, 5, and 13 act to block the penetration of high frequency signals.
In this amplifier, the gain control is performed by adjusting the amount of attenuation, and its functional block intended as an amplifier for raising the gain is not shown in the figure. Therefore, as far as the circuit of FIG. 19 is concerned, the circuit functions as an attenuator.
FIG. 20 shows characteristic diagrams illustrating how the gain control is accomplished in the amplifier of FIG. 19 when the threshold voltage Vth of each of the field effect transistors 6, 1, and 9 is -1.0 V. FIG. 20(a) shows the gain control voltage Vc versus gain (amount of attenuation) characteristic of the field effect transistor (series FET) 6 forming the series variable resistor 71. FIG. 20(b) shows the gain control voltage Vc versus gain (amount of attenuation) characteristics of the field effect transistors (shunt FETs) 1 and 9 forming the parallel variable resistors 72 and 73; the solid line shows the characteristic of the two transistors combined, and the dashed line the characteristic of either alone. FIG. 20(c) shows the gain control voltage Vc versus gain (amount of attenuation) characteristic of the amplifier of FIG. 19, obtained by combining the characteristics of FIGS. 20(a) and 20(b).
When the threshold voltage Vth of each of the field effect transistors 6, 1, and 9 is -1.0 V, as stated above, for the field effect transistors 1 and 9 forming the parallel variable resistors 72 and 73 the gain (amount of attenuation) varies over a range of 14 dB with a slope of 46 dB/V in proportion to the applied gain control voltage Vc when the gain control voltage Vc is varied within the range of 0.7 V to 1.0 V, while for the field effect transistor 6 forming the series variable resistor 71, the gain (amount of attenuation) varies over a range of 15 dB with a slope of 50 dB/V in proportion to the applied gain control voltage Vc when the gain control voltage Vc is varied within the range of 2.0 V to 2.3 V; this means that, within the voltage range of 1.0 V to 2.0 V, gain control is not performed but the gain (amount of attenuation) is maintained constant irrespective of the variation of the applied gain control voltage Vc. That is, gain control voltage section .DELTA.V where no gain control is performed is as wide as 1.0 V.
FIG. 21 shows characteristic diagrams illustrating how the gain control is accomplished in the amplifier of FIG. 19 when the threshold voltage Vth of each of the field effect transistors 6, 1, and 9 is -2.0 V. FIG. 21(a) shows the gain control voltage Vc versus gain (amount of attenuation) characteristic of the field effect transistor (series FET) 6 forming the series variable resistor 71. FIG. 21(b) shows the gain control voltage Vc versus gain (amount of attenuation) characteristics of the field effect transistors (shunt FETs) 1 and 9 forming the parallel variable resistors 72 and 73; the solid line shows the characteristic of the two transistors combined, and the dashed line the characteristic of either alone. FIGS. 21(a) and 21(b) are the same as FIGS. 20(a) and 20(b). FIG. 21(c) shows the gain control voltage Vc versus gain (amount of attenuation) characteristic of the amplifier of FIG. 19, obtained by combining the characteristics of FIGS. 21(a) and 21(b).
When the threshold voltage Vth of each of the field effect transistors 6, 1, and 9 is -2.0 V, as stated above, for the field effect transistor 6 forming the series variable resistor 71 the gain (amount of attenuation) varies over a range of 15 dB with a slope of 50 dB/V in proportion to the applied gain control voltage Vc when the gain control voltage Vc is varied within the range of 1.0 V to 1.3 V, while for the field effect transistors 1 and 9 forming the parallel variable resistors 72 and 73, the gain (amount of attenuation) varies over a range of 14 dB with a slope of 46 dB/V in proportion to the applied gain control voltage Vc when the gain control voltage Vc is varied within the range of 1.7 V to 2.0 V; this means that, within the voltage range of 1.3 V to 1.7 V, gain control is not performed but the gain (amount of attenuation) is maintained constant irrespective of the variation of the applied gain control voltage Vc. That is, gain control voltage section .DELTA.V where no gain control is performed is as wide as 0.4 V.
In the amplifier having the above characteristics, when performing step control, the gain control voltage Vc is varied in steps to vary the gain (amount of attenuation) incrementally by a predetermined amount, and gain control having good linearity (within .+-.1 dB), for example, over a wide gain control range of 70 dB, is accomplished by combining such step control with the continuous gain control performed at intermediate frequencies.
FIG. 22 shows an example of a step control characteristic diagram for the amplifier, such as described above, that performs gain control in the high frequency section. Usually, the number of gain steps is chosen to be about 2 to 10; in FIG. 22, the gain control width of 30 dB is divided into three steps in increments of 15 dB, i.e., a low mode, a middle mode, and a high mode in increasing order of the gain. Symbols V.sub.CL, V.sub.CM, and V.sub.CH denote gain control voltages corresponding to the respective modes, and symbol P.sub.OUT indicates the magnitude of the output signal. In FIGS. 20 and 21, the gain control width has been shown as being 29 dB, but here the width is rounded up to 30 dB for simplicity of calculation.
On the other hand, for the amplifier (intermediate frequency amplifier) that performs continuous gain control at intermediate frequencies and is used in combination with the amplifier (high frequency amplifier), such as shown in FIG. 19, that performs the steplike gain control, an amplifier using a silicon bipolar transistor is the predominant type, though its specific circuit diagram is not shown here.
With the amplifier that performs gain control using a bipolar transistor, gain control having excellent linearity can be achieved over a relatively wide range at small signal levels because of the characteristic of its circuit configuration. Accordingly, the amplifier used to perform gain control at intermediate frequencies performs continuous control using the characteristic that the gain changes linearly.
FIG. 23 is a continuous control characteristic diagram for the amplifier that performs gain control at intermediate frequencies. As shown in FIG. 23, gain control with excellent linearity of .+-.1 dB flatness over a relatively wide gain control range of 40 dB or greater can be performed at a small signal level; thus, continuous control is performed at intermediate frequencies for the respective modes in the step control performed at high frequencies. Symbol V.sub.CFINE denotes a fine adjusting gain control voltage for the respective modes, and symbol P.sub.OUT indicates the magnitude of the output signal.
Further, FIG. 24 is a gain control characteristic diagram for the mobile communication terminal transmitter, showing the characteristic of the step control combined with the continuous control. By combining the respective modes of the step control in FIG. 22 with the continuous control of FIG. 23, gain control having excellent linearity with a flatness of .+-.1 dB can be performed over a wide gain control range of 70 dB or greater at the mobile communication terminal transmitter.
As described above, to achieve gain control having a wide gain control range of 70 dB or greater and excellent linearity with a flatness of .+-.1 dB in the mobile communication terminal transmitter, it has been practiced in the prior art to manufacture separately a device for performing step control at high frequencies and a device for performing continuous control at intermediate frequencies, and to connect the two kinds of amplifiers (semiconductor devices), constructed by packaging the respective semiconductor chips, using an external circuit.
The reason for this is that in the prior art configuration described above, since only one stage of series variable resistor 71 formed from the field effect transistor 6 is connected between the input terminal 14 and the output terminal 15, the linear portion of the gain control amount that changes as a function of control voltage cannot be made wider than about 15 to 18 dB and the gain control width is also as small as about 30 dB, and therefore that gain control having a wide gain control range of 70 dB or greater and excellent linearity with a flatness of .+-.1 dB is difficult to achieve with a single amplifier in the high frequency section of the mobile communication terminal transmitter.
However, using two separate kinds of amplifiers (semiconductor devices) for performing separate gain controls, as described above, has had the following problems.
The first problem is that when the amplifier for performing gain control in the high frequency section is used to perform step control, gain control voltage dead zone .DELTA. where the gain control cannot be performed occurs between the gain control voltage range where the series variable resistor 71 performs a linear gain control operation and the gain control voltage range where the parallel variable resistors 72 and 73 perform a linear gain control operation; accordingly, when changing the gain (amount of attenuation) in steplike manner by changing the gain control voltage Vc in steplike manner, it has been difficult to accomplish highly precise gain control, even if the gain control voltage is changed precisely with respect to the gain (amount of attenuation). In other words, there has been the problem that if precise adjustment of the gain (amount of attenuation) is to be achieved, the selection of the gain control voltage Vc will become complex.
A second problem is that if different kinds of gain controls (step control and continuous control) are used in combination, that is, if different kinds of gain controls (step control and continuous control) are performed using two different kinds of amplifiers (semiconductor devices), since the setting of the gain control voltage Vc in the continuous control is changed at the same time that the mode switches from one mode to the next in the step control, as can be seen from the gain control characteristic diagram of FIG. 24, gain differences occur before and after the mode switching, and the desired flatness characteristic cannot be achieved, thus making it difficult to increase the precision of the gain control. Furthermore, two kinds of control voltage settings become necessary in the microcomputer logic blocks, which adds to complexity. There has also been the problem that the circuit configuration of the mobile communication terminal becomes complex and increases in size.